Eight-arm star polystyrene in methylcyclohexane: Cloud-point curves, critical lines, and coexisting densities and viscosities

We measured the cloud‐point curves of eight‐arm star polystyrene (sPS) in methylcyclohexane (MCH) for polymer samples of three total molecular masses [weight‐average molecular weight (Mw) × 10−3 = 77, 215, or 268]. We found a downward shift of 5–15 K in the critical temperature (Tc) of the star polymer solutions with respect to linear polystyrene (PS) solutions of the same Mw. The shift in Tc became smaller as Mw increased. The critical volume fraction for eight‐arm sPS in MCH was equal within experimental uncertainty (10–40%) to that of linear PS in MCH. For sPS of Mw = 77,000 in MCH, we studied the mass density (ρ) as a function of temperature (T). As for linear polymers in solution, the difference in ρ between coexisting phases (Δρ) could be described over t = (Tc − T)/Tc for 1.1 × 10−4 < t < 4.7 × 10−3 with the Ising value of the exponent β in the expression Δρ = B tβ. Both ρ(T) above Tc and the average value of ρ below Tc were linear functions of temperature; no singular corrections were observed. The measurements of the shear viscosity (η) near Tc for sPS (Mw = 74,000) in MCH indicated a strong critical anomaly in η, but the data were not precise enough for a quantitative analysis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 129–145, 2004